This invention relates to the art of intrusion monitoring and more particularly to underwater echo detection of objects propelled with a patterned cadence (e.g., a human swimmer).
Prior underwater intrusion monitoring apparatus reply upon detection and analysis of variations in the received echoes of a continuous-wave signal transmitted by either scanning or rotating a narrow beam width signal through the volume to be monitored. Although a narrow beam signal has a higher signal-to-noise ratio than a wide beam signal, in order to monitor the same volume of water as a wide beam device, a narrow beam device must rely upon a longer beam length. A strong thermal gradient causes the upper surface of a beam to bend away from the surface of the water, an effect that becomes more pronounced with the beam length. The thermal gradient effect means that an effective beam of limited length is obtainable during the afternoon hours on a warm day while an effective beam of significantly greater length is obtainable with the same device during the cooler night hours on the same day. It is therefore apparent that although deployed prior art devices provide an adequate monitoring of a volume of water during the nighttime hours, they are unable to monitor the more distant majority of that volume during the afternoon hours due to bending of the upper edge of the beam lobe, thus leaving a significant gap in the monitored volume in which the relatively quiet propulsive movements of a submerged intruder during the traditional hours of peak surface activity (e.g., small vessels entering or departing a harbor as well as the less frequent movements of larger vessels) are unnoticeable. Additionally, their dependency upon electrically switched scanning or mechanical rotation subjects a prior art intrusion monitoring device to an increased risk or failure due to the inherent weakness of its mode of operation.